Sensory Transduction by Hair Cells of the Inner Ear
Summary: Jim Hudspeth studies the biophysical and molecular bases of hearing and equilibrium. In particular, he is interested in the development, regeneration, and operation of the inner ear's sensory receptors, the hair cells.
Forty million Americans have significant hearing problems that range in severity from modest difficulty with speech comprehension to profound deafness. Hearing impairment affects people of all ages. One child in a thousand is born deaf, for example, and another person in a thousand becomes deaf before adulthood. A quarter of our population over 60 years of age is hearing-impaired, and half of those older than 80 are afflicted.
Most hearing-impaired individuals suffer from sensorineural hearing loss that results from damage to hair cells, the sensory receptors of the inner ear. Each cochlea normally contains about 16,000 hair cells, which convert mechanical inputs derived from sounds into electrical signals that the brain can interpret. Similar cells in the vestibular labyrinth mediate responsiveness to accelerations and thus underpin our sense of balance. Human hair cells may be lost throughout life as a result of genetic conditions, infections, ototoxic drugs, acoustical trauma, and ageing. Because these cells are not replaced by cellular division, their disappearance is associated with a gradual decline of our senses of hearing and equilibrium. In the hope of understanding normal hearing—as well as its development, its deterioration, and its possible restoration—our group is investigating the molecular structure and operation of hair cells from the vertebrate internal ear.
Active Hair-Bundle Motility
We benefit from an active process that accounts for the extraordinary sensitivity of hearing, which is nearly a thousandfold that expected for a passive system. Frequency tuning, too, is enhanced by the active process. This phenomenon also accounts for the broad dynamic range of hearing: We can analyze sounds whose amplitudes vary by a millionfold. Amazingly enough, exuberant activity by the amplifier can even result in the production of sounds by the ear, the so-called spontaneous otoacoustic emissions. At the opposite extreme, damage to the auditory amplifier underlies the commonplace deterioration of human audition—"hardness of hearing"—as a consequence of noise exposure, use of certain drugs, or aging.
One component of the active process involves active motility by the hair bundle, the hair cell's mechanically receptive organelle. A hair bundle consists of an upright array of enlarged stereocilia that is deflected by sounds, thus opening transduction channels and initiating an electrical response. The bundle can oscillate in the absence of inputs, a possible source of spontaneous otoacoustic emissions. More importantly, the bundle can amplify its mechanical inputs, thus augmenting the sensitivity of hearing. Using a unique double-beam laser interferometer that can detect internal motions of the hair bundle with a precision below 1 nm and a bandwidth above 10 kHz, we have demonstrated that all the stereocilia in a bundle move in a coordinated fashion. This result supports our hypothesis that active hair-bundle motility stems from concerted gating of the transduction channels, which introduces negative stiffness into the bundle's behavior.
Coordination of Active Processes in the Cochlear Amplifier
We have known since the investigations of von Békésy in the 1950s that sound excites a traveling wave that propagates along the elastic basilar membrane from base to apex and delivers sound energy to the mechanoreceptive hair cells. Over the past decade, researchers have extended this understanding by demonstrating the role of the cochlea's active process in shaping the traveling wave and thereby producing amplification, sharpened frequency tuning, and compressive nonlinearity. It remains unclear, however, precisely how the active process operates. On the one hand, active hair-bundle motility demonstrably powers the active process in nonmammalian tetrapods as well as in mammals. On the other hand, mammalian outer hair cells display membrane-based somatic motility that operates to remarkably high frequencies and is implicated by transgenesis experiments in cochlear amplification. This phenomenon reflects the activity of the protein prestin, which is present in millions of copies in the basolateral cell membrane and exhibits piezoelectric alterations in its dimensions in response to changes in membrane potential.
We have devised strategies to examine the roles of the two motile processes by selective inactivation of one or the other. We found that 4 azidosalicylate is an effective blocker of prestin's activity that can be covalently linked to the protein by ultraviolet light. By inactivating prestin over short segments of the basilar membrane, we confirmed that somatic motility amplifies the traveling wave as it approaches its peak. In complementary experiments, we used a variety of pharmacological treatments to modulate active hair-bundle motility. This approach demonstrated that the process plays a key role in poising the cochlea near a dynamical instability, the Hopf bifurcation, at which the active process is most effective.
The complexity and inaccessibility of the cochlea restrict in situ experiments on hair cells. To overcome this limitation, we have conducted extensive mathematical analysis and modeling of hair cells and the cochlea. We found that the hair bundle's activity can be greatly augmented by loading with a suitable mass, a proposition that we are now testing experimentally. The models additionally disclosed the sensitivity of active bundle motion to the stiffness of the external load and to the application of force. In experiments now in progress, we are testing this prediction by use of a displacement-clamp system. Finally, modeling indicated how hair-bundle motility and somatic motility collude to produce the remarkable features of the active process.
Regeneration of Hair Cells in the Zebrafish Lateral Line
Although hearing aids and cochlear prostheses provide partial relief from hearing loss, the most promising avenue to the alleviation of deafness lies in the regeneration of hair cells. Therefore, our group is investigating the mechanism of regeneration in the lateral line of the zebrafish, a model system in which new hair cells are formed continually throughout life. After developing lines of transgenic fish in which various fluorescent proteins are expressed in specific cell populations, we have used fluorescence-activated cell sorting and whole-transcriptome shotgun sequencing to ascertain which genes display increased or decreased expression during the course of hair-cell regeneration. In situ hybridization has confirmed the cell-specific expression of the relevant proteins, whose roles are now being tested in knockdown experiments.
These experiments were supported in part by grants from the National Institutes of Health.
As of March 11, 2013